First off, Unrelenting devestation is a special ability (much like wrathful descent). In the same vein as the magnitude damage from wrathful is not caused by the close combat attack, the magnitude damage from unrelenting devestation is not technically caused by attack. It is tacked on AFTER all other considerations from the attack.
An X weapon causes 1 additional hit per attack (BS roll). This additional hit is still considered from the weapon fired and is a normal part of the attack being made. Therefore it will be doubled by the Storm of Iron talent as normal.
Hits from Metal Storm rounds probably should be doubled from Storm of Iron as the shots are being fired with a semi or full auto burst (which, besides flamer, is the only requirement or limitation for Storm of Iron).
So a Heavy Bolter hitting 3 times follows the following course of action when dealing damage.
3 hits from BS
+1 hit from X type
4 total hits
Roll damage for each hit: all 4 deal 1+ damage for 4 magnitude damage
Double the attacks magnitude damage with Storm of Iron: Mag damage is now 8
Add 1d5 (rolled a 3) extra magnitude damage from Unrelenting Devestation
In this example the Devastator did a total of 11 magnitude damage.
Damage from hits is not automatic, although you may skip rolling if minimum damage is above the target's Armour+Toughness.
Any hit which failed to do 1+ damage will not damage magnitude and therefore will not be doubled with Storm of Iron.
Unrelenting Devestation damage is not doubled with Storm of Iron as it is not damage originating from the actual attack.
Storm of Iron + Metal Storm can be a little painful but here are a few things which may help you out as a GM with creating challenging encounters.
1: All special issue ammunition for heavy bolters cost +5 requisition for a standard 60 round magazine and +15 requisition for a Backpack ammo supply. So Metal Storm rounds for a heavy bolter costs 20 req for a 60 round mag, and 30 req for a backpack.
2: The Tactical Marine's "Free Magazine" of special issue ammunition is not a "Free backpack". It is just a normal magazine (60 rounds).
3: Hordes count as a single character (albiet made up of many individuals) and may use any combat actions or tactics a normal character can. This means they may use actions such as Defensive Stance (though they don't benefit from additional reaction) or Tactical Advance. They can also benefit from cover (really consider using it, it will make weak characters suprisingly tough to kill) and other special rules.
4: Use a leader (such as a Lieutenant, sergeant, etc.) to issue commands or special actions to their troops. For instance, if 5 infantry (not a horde) are nearby have the lieutenant use the command skill to order them to 'concentrate fire' on one target, counting as a horde for that round of shooting.
5: Deffinately check out the advanced horde rules in Mark of the Xenos.
P.S. My enter key will not work while I was typing this up in the board (did it in wordpad and copy/pasted). FFG! Set my forumites free!… of this junky forum programming.